This invention relates to a dielectric forming material for use in a plasma display panel and a glass powder used as a main component thereof.
In general, a plasma display panel (hereinafter which will be also referred to as "PDP") comprises a pair of glass substrates on which electrodes for plasma discharge are formed. More specifically, PDPs are classified into DC type PDPs and AC type PDPs. The DC type PDP has electrodes which are exposed to a discharge space while the AC type PDP has electrodes which are coated with an insulating layer.
The AC type PDP comprises a front glass substrate and a rear glass substrate which are opposed to each other with a gap left therebetween. The front glass substrate and the rear glass substrate have first and second principal surfaces, respectively, at opposite sides. A plurality of display electrodes are formed on the first principal surface of the front glass substrate in strips. Each display electrode extends in a predetermined direction. A dielectric layer is formed on the first principal surface of the front glass substrate so as to cover the display electrodes therewith. The dielectric layer is used to maintain discharge. A protection layer is deposited on the dielectric layer by evaporation.
On the other hand, a plurality of address or data electrodes are formed on the second principal surface of the rear glass substrate in strips. Each address electrode extends in a direction which is perpendicular to the predetermined direction. A plurality of barrier ribs are formed on the second principal surface of the rear glass substrate in strips so that the barrier ribs are put between the address electrodes. In other words, two adjacent address electrodes are separated by one barrier ribs. In addition, phosphors of red (R), green (G), and blue (B) are deposited on the second principal surfaces of the rear glass substrate between the barrier ribs so as to cover the address electrodes therewith. Like in a cathode ray tube (CRT), the phosphors of three primary colors (RGB) compose one pixel.
Discharge gas is enclosed in the space between the front and the rear glass substrate with the circumference sealed tightly. The discharge gas generates a lot of ultraviolet rays on discharging.
With this structure, when a voltage is applied between a selected one of the display electrodes and a selected one of the address electrodes, gas discharge occurs at a point of intersection between the selected display electrode and the selected address electrode to generate the ultraviolet rays from the point of intersection. The ultraviolet rays excite the phosphors formed in the panel to make the phosphors emit visible light.
It is necessary for the dielectric layer to have a high dielectric strength and to have good transparency.
In prior art, such a dielectric layer is formed by a screen printing method which comprises the steps of carrying out screen-printing on a paste dielectric forming material containing glass powder and of firing the paste dielectric forming material.
However, in the screen printing method of forming the dielectric layer, marks of screen mesh remain on a surface of the dielectric layer after the screen-printing. As a result, the screen printing method is disadvantageous in that it is difficult to obtain a smooth surface of the dielectric layer, the dielectric layer has an unstable thickness, a lot of bubbles remain in the dielectric layer, and so on. In addition, the screen printing method is also disadvantageous in that it is difficult to obtain the high dielectric strength and to ensure sufficient transparency. Furthermore, in order to obtain the dielectric layer having a sufficient thickness, for example, of about 30 to 40 .mu.m, the screen printing method must be repeatedly carried out about three to five times. Accordingly, evaporation of a solvent for use in drying process carried out each screen-printing results in easily incurring atmospheric pollution of the work environment in the screen printing method.
In order to resolve the above-mentioned problems in the screen printing method, another method of forming the dielectric layer is proposed in Japanese Unexamined Patent Publication No. Hei 9-102273, namely, JP-A 9-102273. The other forming method disclosed in JP-A 9-102273 comprises the steps of preparing a green sheet composed of dielectric material and of firing the green sheet with the green sheet attached to the front glass substrate on which the display electrodes are formed. Such a forming method is herein referred to as a green sheet method. The green sheet is made by applying paste-form composition containing glass powder, resin, and a solvent, on a support film consisting of the film of polyethylene terephtalate.
The green sheet method is advantageous in that it has an excellent workability and it is difficult to incur atmospheric pollution of the work environment. In addition, inasmuch as slurry used to sheet forming has a lower viscosity than that of paste used to the screen-printing, rolling or wrapping of bubbles is few on the sheet forming. Accordingly, it is possible to obtain the glass film which has a smooth surface and a uniform coating thickness and it is possible to form the dielectric layer having a high dielectric strength.
However, a conventional green sheet method according to JP-A 9-102273 may often obtain the dielectric layer having an insufficient transparency because a lot of microscopic bubbles remain in the dielectric layer.